Wierd errors

Hello! I am trying to code a program that can solve quadratic equations. The problem I seem to be having is that when I try to compile it, it comes up with errors leading to the math.h file. This is the code:

What's the point of if( choice == ... )? You should be using string choice instead if char choice (on line 20) if you want to compare to a string, and 'single quotes' (not "double quotes") if you want to compare to a charecter. But why bother with the comparison at all?